1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. Note that in this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, a transistor (a thin film transistor and the like) can be given. In addition, a semiconductor device also refers to a display device such as a liquid crystal display device.
2. Description of the Related Art
Semiconductor devices have been indispensable to our life. Silicon has been mainly used as conventional semiconductor materials applied to semiconductor devices. However, in recent years, as semiconductors applied to semiconductor devices, oxide semiconductors have attracted attention. Semiconductor devices in which a Zn—O-based metal oxide or an In—Ga—Zn—O-based metal oxide is used as oxide semiconductors are disclosed in Patent Document 1 and Patent Document 2.